1. Field of the Invention
This invention relates to R—Fe—B base sintered magnets containing silicon as additive element.
2. Background Art
Prior art R—Fe—B base sintered magnets, for example, those described in Japanese Patent Nos. 1,431,617 and 1,655,487 are utilized in a variety of applications for their excellent magnetic properties. Typically Nd and Pr are used as the rare earth R, but as such, temperature characteristics are undesirable. Then partial replacement of R by Dy or Tb is employed for increasing the coercive force at room temperature as disclosed in Japanese Patent No. 1,802,487.
R—Fe—B base sintered magnets are structured such that a hard magnetic phase of R2Fe14B is present as a primary phase, and grain boundary moieties surround primary phase grains. The grain boundary moieties are composed of an R-rich phase (a phase containing 80–98 at % R) and a phase represented by the composition R1+εFe4B4 (ε=0.1 in the event R=Nd) or R2Fe7B6, known as B-rich phase. The structure further includes oxide, carbide and other phases which are inevitably introduced by the manufacturing process.
It is also known that various elements when added form compound phases such as RM2, R3M and R5M3 wherein M is an additive element.
One of the additive elements commonly added to Nd magnets is silicon. See Japanese Patent Nos. 2,138,001, 1,683,213, 1,737,613, and 2,610,798, JP-A 60-159152 and JP-A 60-106108. In these patents, silicon is added mainly for the purposes of improving temperature characteristics or oxidation resistance.
As to the addition of Si to Nd magnets, it is known that the extent of improvement is not so great when added in trace amounts, whereas addition of 1% or more can degrade the magnetic properties such as Br and iHc.
As mentioned above, heavy rare earths are often used for increasing the coercive force. Since the heavy rare earths such as Dy and Tb are present in less reserves in the crust than light rare earths, their cost is very high as compared with Nd. The coercive force increases with the increasing amount of Dy or Tb added, but the material cost increases at the same time. As the magnet market will expand from now on, magnets containing high concentrations of Dy and Tb will become in short supply, which poses a problem.
A study is thus made on additives other than Dy and Tb as another means for increasing coercive force.
Of other additives, V, Mo, Ga and the like have been reported to have a coercive force increasing effect. However, they belong to the rare metal family and offer little advantages as the replacement for Dy.
In order that R—Fe—B base magnets adapted for high-temperature use find a large market in the future, it is requisite to have a novel method or magnet composition that can increase the coercive force while minimizing the amount of Dy added.